Gold Dragons are only immune to magic, and I am pretty sure they are immune to finger of death. Of all the damage types I mentioned, I am most sure about shrapmetal (I am very certain it deals physical damage) and finger of death (instant death effects depends on magic resistance according to grayface at https://grayface.github.io/mm/mechanics/).

I am pretty sure that finger of death will allow monsters to negate the effect through resistance but not their luck stat (monsters don't have luck stat). According to Grayface, monsters with 70 magic resistance will have a 70% chance of negating damage, meaning that 20 points in dark magic will leave you with a 30% chance of working. This would mean that you would need a dark magic skill of 67 to insta-kill a monster with 70% magic resistance.

I would agree that finger of death is unreliable, and I also feel that a skill of 67 sounds a little low to insta-kill anything not immune to magic (it's not that hard to get such a high skill with "of dark magic" items). Someone else would need to confirm.

EDIT:
Typo

if it dealt physical damage it wouldn't be able to kill slimes and it very much is able to kill slimes. as for the finger of death magic resist thing at least in vanilla might and magic skills are capped at 60 even with item boosts but from personal experience the chance to 1 shoot high level enemies with it is reaaally low even when you have 60 in the skill

Shrapmetal can not kill slimes. I litterally checked this on my MM6 (non-merge mod) save 30 seconds ago.

EDIT:
Typo again.

I found this so unbelievable that I actually went ahead and tested it and you are completely right. its interesting that they changed it in the later games though, kind of a cool touch that summoning metal dealt physical damage in MM6.

EDIT: also tested the 60 dark magic mastery finger of death on the slimy boys and didn't manage to kill any over multiple tries lol

I guess I thought the complete opposite. Since I knew that physical immunes couldn't take damage from shrapmetal in MM6, I guess I never tried hurting them with it in MM7-8.

Tested all 3 games now, and in MM7 and MM8 you can indeed kill oozes with shrapmetal (tested on fears in the plane between planes in MM8 since they are physical immune and MM8 has no oozes AFAIK).

There's no good reason to use anything other than casters in MM6 since weapons deal pitiful damage compared to spells and warrior classes get no special bonuses. They get somewhat higher hitpoints but compared to the hitpoints available to healers its nothing. So even if you want to make a melee party you're better of arming healers.

there isnt much research required for any class but there is a choice of early and lategame power. You dont really need anything early game other than bows but there will be some difficult fights especially if you're still learning the game.
if you clear most of the game you will reach level 80+ before the end of the game however, so you can easily invest in some early game skills

Fire magic has great AOE spells; fireball, ring of fire available in mist which you can reach from starting town by boat.
body magic has great healing spells
between the two early game can be very easy indeed

late game you'll want to deal physical damage making your choice basically shrap metal or implosion. monsters have resistance to other types of damage, physical damage spells are effective vs almost all enemies.
shrap metal deals far more damage but is basically a melee range spell, implosion is ranged and has the advantage it can be used by druids

I'd argue the easiest and possibly best group is 3 druids+1 cleric you get a high damage output, ranged. up to 3 AOE casters and all of them able to cast healing spells - body magic requires lots of skillponts to be effective but having every charater able to cast raise dead and town portal makes you basically immortal.
shrap metal does deal more damage though, so in a way its the better late game spell if risky. Honestly it deals too much damage for the game to be fun though - while being risky in some of the toughest fights vs many powrful enemies because it needs melee...and going melee is not a very fun way to play the game imo, dodging attacks and ifring at range is much more fun.

Early game you get fire and body magic, so its also the easiest group imo.

1 body magic for main healer (cleric)
2 body/fire magic leveled in tandem for secondary healer
3 Fire for main AOE
4 Fire for secondary AOE (if you cheat with easy quests this one gets water magic asap instead)

(3 and 4 still get body magic but dont level it. first aid is good when needed even at 1 skilllevel)

get water magic to master level, body to master for for both healers, fire to skill level 7 to 12 for 3 & 4

when thats done you start getting most magic skills to 4 for at least 2 casters; you want at least 2 with expert spirit to cast raise dead, two casters with expert earth to cast stone to flesh etc
cleric gets dark magic to level 4-12 for day of protection
you want all druids to get expert fire and ring of fire and inferno for when fighting many enemies
you also get meditation and bodybuilding to expert for all your guys.

then start leveling air magic as high as you can and use implosion/lightning bolt as main spell, and fire for AOE.
your main healer, the cleric just levels bodymagic. secondary healer keeps leveling air magic in tandem with body magic - maybe keep body magic a bit lower but dont neglect it, you are going to want a secondary healer occasionally